A large variety of processes and catalysts exist for the homopolymerization or copolymerization of olefins. For some applications, it is desirable for the polyolefin product to have a low weight average molecular weight (less than 50,000 g/mol) combined with a relatively narrow and controllable molecular weight distribution, since such polymers can be blended with higher molecular weight LLDPE containing hexane or other comonomers to produce blends with enhanced impact strength, high environmental stress crack resistance and good processing characteristics.
Traditional Ziegler-Natta catalysts systems comprise a transition metal compound co-catalyzed by an aluminum alkyl but typically produce polyolefins of high molecular weight, generally with a relatively broad molecular weight distribution.
More recently metallocene catalyst systems have been developed wherein the transition metal compound has one or more cyclopentadienyl, indenyl or fluorenyl ring ligands (typically two). Metallocene catalyst systems, when activated with cocatalysts, such as alumoxane, are effective to polymerize monomers to polyolefins having a wide range of average molecular weights and a narrow molecular weight distribution. However, the target weight average molecular weights for gas phase or solution polymerization using metallocene catalyst are generally in the range of 80,000 to 120,000 g/mol. Moreover, metallocene catalysts frequently suffer from a number of disadvantages, for example, high sensitivity to impurities when used with commercially available monomers, diluents and process gas streams, the need to use large quantities of expensive alumoxanes to achieve high activity, and difficulties in putting the catalyst on to a suitable support.
There is therefore significant interest in developing new catalyst systems for the homopolymerization or copolymerization of olefins.
In 1998 two separate groups working independently reported that bis(imino)pyridyl iron complexes of the type {2,6-[ArN═C(Me)]2C5H3N}FeCl2 can be activated with methylalumoxane (MAO) to produce highly active catalysts for ethylene polymerization. See B. L. Small et al., J. Am. Chem. Soc. 120 (1998), 4049 and G. J. P. Britovsek et al., Chem. Commun. (1998), 849.
U.S. Pat. No. 6,451,939 discloses catalyst systems for the polymerization or copolymerization of 1-olefins which contain nitrogen-containing transition metal compounds comprising the skeletal unit depicted in the following formula:
wherein M is Fe[II] Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III] or Ru[IV]; X is an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; and R1 to R7 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl. The transition metal compound of this formula can be unsupported or supported on a support material, for example, silica, alumina, or zirconia, or on a polymer or prepolymer, for example polyethylene, polystyrene, or poly(aminostyrene).
In addition, U.S. Patent Application Publication No. 2003/0104929 (equivalent to WO 01/23396) discloses a nitrogen containing transition metal complex having formula:
wherein M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III] or Ru[IV]; X represents an atom or group bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1, R2, R3, R4, R5, R19, R20, R21, R22, R23, R24, R25, R24, R26 and R28 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; provided that when any two or more of R1, R2, R3, R4, and R5 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, two or more can be linked to form one or more cyclic substituents, and at least one of R4 and R5 is a hydrocarbyl group having at least two carbon atoms. The transition metal complex of this formula can be unsupported or supported on a support material, for example, silica, alumina, MgCl2 or zirconia, or on a polymer or prepolymer, for example polyethylene, polypropylene, polystyrene, or poly(aminostyrene), and can be used in an olefin polymerization catalyst system.
Although bis(imino)pyridyl transition metal complexes have been shown to be highly active ethylene polymerization catalysts when activated with MAO or even some aluminum alkyl co-catalysts, they produce polyethylene with a broad molecular weight distribution when run as a homogeneous (unsupported) system. Moreover, most attempts to employ supported bis(imino)pyridyl transition metal complexes have only resulted in further broadening of the polymer molecular weight distribution. There is therefore a need to develop catalysts systems which contain supported bis(imino)pyridyl transition metal complexes and which are effective in producing polyolefins with a narrower molecular weight distribution.
In Journal of Catalysis, 234 (2005), 101-110, Zheng et al. report that the modification of bis(imino)pyridyl ligands by the introduction of reactive ethoxysilane or Si—Cl end groups, allows the ligands to be immobilized on silica by direction reaction of the ethoxysilane or Si—Cl groups with silanol groups on the silica surface. Complexation of the supported ligands with FeCl2.4H2O is said to produce silica-supported Fe(II) precatalysts, which exhibit high activity for ethylene polymerization in the presence of modified methylalumoxane (MMAO). In one case, using the precatalyst obtained from the ethoxysilane-modified bis(imino)pyridyl ligands, polymerization produced polyethylene having a weight-averaged molecular weight of 50.1×104 g/mol and a unimodal molecular weight distribution with a relatively high polydispersity index of 9.7.
In Journal of Molecular Catalysis A: Chemical, 260 (2006), 135-143, Huang et al. report that ethylene polymerizations carried out with various bis(imino)pyridyl iron, chromium and vanadium complexes immobilized on a support of the type MgCl2/AlRn(OR)3-n gave polyethylene of relatively broad molecular weight distribution, but high molecular weight, in the case of the iron complex, but gave a very narrow molecular weight distribution in the case of the vanadium complex.
In Macromol. Chem. Phys., 207, (2006), 779-786, Xu et al. report that spherical MgCl2 supports obtained by the thermal dealcoholization of MgCl2.2.56C2H5OH at 170° C. for 4 hours are effective for immobilizing bis(imino)pyridyl FeII and NiII complexes. When the resultant nickel complexes were used to polymerize ethylene in the presence of triisobutylaluminum as activator, the polyethylene products exhibited a weight-averaged molecular weight between 44 and 90×104 g/mol and a molecular weight distribution between 2.35 and 2.87. In the case of the iron complexes, ethylene polymerization in the presence of triethylaluminum as activator produced polyethylene products exhibiting a weight-averaged molecular weight between 49 and 61×104 g/mol and a molecular weight distribution between 11.7 and 20.
It has now been found that supports, such as silica, having an aluminum halide and/or an alkylaluminum halide chemically or physically bonded thereto are effective in immobilizing bis(imino)pyridyl transition metal complexes and that the resultant supported complexes, when combined with an activator, such as a perfluorinatedaryl borate, and optionally an trialkylaluminum co-catalyst, are effective in polymerizing α-olefins, such as ethylene. In particular, with suitable selection of the ligand structure of the complex, the supported catalysts can be used to produce polymers of low weight average molecular weight combined with a relatively narrow molecular weight distribution.